Win32: Add comment clarifying use of ToUnicode

Fixes #2100

src/win32_init.c

@@ -502,6 +502,8 @@ void _glfwUpdateKeyNamesWin32(void)
 
         if (length == -1)
         {
+            // This is a dead key, so we need a second simulated key press
+            // to make it output its own character (usually a diacritic)
             length = ToUnicode(vk, scancode, state,
                                chars, sizeof(chars) / sizeof(WCHAR),
                                0);